Reducing the quantity of water in formulations containing hydraulic binders, though without altering their fluidity, is essential: this results in doping their mechanical properties. To do so, the person skilled in the art has for several years developed admixtures known as “water-reducing agents”, also known by the expressions “fluidifying agents”, “plasticizers”, and “superplasticizers”.
Historically, the first of them were lignosulfonates, as described in the document U.S. Pat. No. 3,772,045. Polycondensates of formaldehyde and naphtalene or melamine sulfonates were then used, as illustrated in the documents U.S. Pat. No. 3,359,225 and U.S. Pat. No. 4,258,790.
Once adsorbed onto the surface of the cement particles, these polymers, which are negatively charged, cause a phenomenon of electrostatic repulsion, which is the impetus for the cement particle dispersion mechanism; they require a dosage of about 0.4% solids content of the polymer in relation to the solids content of the cement, as instructed in the document “Superplasticizers for extending workability” (International Conference on superplasticizers and other chemical admixtures in concrete, Sorrento Italy, Oct. 29-Nov. 1, 2006, supplementary paper, Ed. Malhotra, American Concrete Institute, pp. 263-277)
A new better-performing family of water-reducing agents then appeared: that of carboxylic comb-branched polymers with a skeleton that is generally (meth)acrylic in nature, onto which are grafted side-chains terminated by hydrophilic groups (see the document cited above).
This improved water-reducing power is explained by the existence of a steric repulsion mechanism relating to the presence of the side-chains, in combination with the electrostatic repulsion phenomenon caused by the anionic carboxylic groups. This improvement leads to the ability to use a lower quantity of the polymer (on the order of 0.2% solids content in relation to the cement's solids content) for a consistency comparable to that obtained with the first-generation products.
However, in dosages like the ones described above, none of these products of the prior art make it possible to resolve a critical technical problem for the person skilled in the art: that of workability retention over very long periods, particularly longer than 3 hours.
Workability is defined in document U.S. Pat. No. 7,232,875 as the time during which an aqueous formulation with a hydraulic binder base may still be worked. This value is conventionally measured using a slump test: the height is determined at which a sample added to a conical mold slumps when that mold is lifted. The greater the slump, the better the formulation's workability; if this nature is maintained over time, this is called slump retention.
The person skilled in the art seeks to improve the lasting nature of this workability. Indeed, it is common to manufacture a cement or concrete in a factory, in order to transport it later to the worksite where it is to be used; during this trip, which may take several hours, the formulation's workability must not be altered; otherwise, it can no longer be worked. Alternatively, a new addition of water-reducing agent must be available at the implementation site (this operation is impossible if the formulation has begun to set), or a series of injections must be available during transport. The logistics and additional cost that result are major drawbacks to using such solutions.
In order to maintain the workability of these formulations, the person skilled in the art has instinctively sought to “overdose” the quantity of water-reducing agents used, and particularly hydrophilic comb-branched (meth)acrylic polymer water-reducing agents. This overdosing is meant to extend the acting time of that agent. However, as is now well-known in this field of activity, this overdosing leads to an initial fluidification which is too high.
Consequently, the fluidifying effect acts against the dispersion mechanism: the distribution of the components is no longer even throughout the formulation, which negatively impacts its mechanical properties. In some cases, the initial fluidifying power is so noteworthy that it leads to segregation, i.e. a physical separation of the medium's various components. This issue is recounted in the document WO 2007/047407.
Given the impossibility of overdosing the water-reducing agents of the prior art, and particularly hydrophilic comb-branched (meth)acrylic polymers, the person skilled in the art has engineered alternative solutions, so as to obtain a satisfactory initial fluidifying power and stable workability over time, without the undesirable effects of segregation or loss of mechanical properties.
Thus, the document WO 2007/047407 suggests the obvious solution of adding a setting retarder agent, which raises problems of its own: the agent in and of itself constitutes a new admixture to add to the formulation, its dosage must be optimized based on the quantity of water-reducing agent used, and the resulting workability is no longer stable past two hours.
Other solutions consist of changes dealing with the composition of the hydrophilic comb-branched (meth)acrylic polymers of the prior art; however, they have the drawback of being focused on highly selective chemical structures, and none of them lead to workability lasting longer than three hours. Furthermore, they are all based on structural changes, intended to extend the fluidifying effect of the polymer that is used over time. To that end, the document “Superplasticizers for extending workability” mentioned above, discloses the grafting of side-chains of varying lengths onto the main (meth)acrylic skeleton: they gradually hydrolyze depending on their length, which maintains the dispersing effect over time (up to two and a half hours according to FIG. 4).
At the same time, the document “Development of new superplasticizer providing ultimate workability” (8th CANMET, Superplasticizers and other chemical admixtures in concrete, 2008, Ed. Malhotra, American Concrete Institute, pp. 31-49) suggests increasing the number of hydrophilic branches of a comb-branched (meth)acrylic polymer by adding a difunctional carboxylic comonomer onto the main skeleton: this results in a better adsorption of the polymer onto the surface of the cement particles, and therefore stable workability for at least one and a half hours according to table 5. Finally, the document “development of slump-loss controlling agent with minimal setting retardation” (7th CANMET, Superplasticizers and other chemical admixtures in concrete, 2003, Ed. Malhotra, American Concrete Institute, pp. 127-141) describes a comb-branched polymer whose hydrophilic side monomers have ester functions which hydrolyze more slowly: this results in good workability for two and a half hours.